1. Field of the Invention
This patent specification relates to a method and apparatus for electrophotographic image forming, and more particularly to a method and apparatus for photographic image forming capable of effectively performing image development.
2. Discussion of the Related Art
In electrophotographic apparatuses such as copying machines, printers, facsimile machines, etc., and particularly small ones, machine maintenance is inevitable and therefore simplification of it is significantly important. For example, published Japanese unexamined patent application, No. 53-3233 (1978), describes a background development apparatus that develops an electrostatic latent image formed on a photosensitive member, a photosensitive sheet, a recording sheet, or the like, with a single-component toner. FIG. 15 illustrates a main portion of a background image forming apparatus using the above-mentioned background development apparatus.
As illustrated in FIG. 15, a photosensitive drum 101 includes a surface layer A and a base B. The surface layer A functions as an image carrying member and the base B functions as a supporting member for supporting the image carrying member. The base B includes a metal drum 102, a relatively-elastic conductive rubber 103 adhered on the surface of the metal drum 102, and a flexible metallic foil 104 (e.g., an aluminum foil) adhered on the rubber 103. The surface layer A includes a photoconductive insulating layer 105 formed by evaporation of a metal such as selenium onto the surface of the metallic foil 104.
A development roller 106 is held in contact with the photosensitive drum 101 under pressure. The development roller 106 includes a metal roller 107 and an elastic layer 108 formed on the surface of the metal roller 107; the elastic layer 108 is made of a conductive substance such as a synthetic rubber, a urethane foam, etc.
In the above-described development apparatus, an elasticizer or a high polymeric substance bleeding out from the above-mentioned elastic layer 108 of the development roller 106 may cause a problem of contaminating the surface layer A of the photosensitive drum 101 or generating a toner tacking or filming phenomenon on the surface of the elastic layer 108 of the development roller 106. For such a problem, a development roller is available in which the elastic layer 108 is coated with a surface layer (not shown) made of resin having a superior releasing property relative to the toner so that bleeding-out of the elasticizer and the high polymeric substance, as well as the generation of toner tacking and filming, can be avoided.
Further, in FIG. 15, a bias power 109 is connected between the development roller 106 and the base B of the photosensitive drum 101. Above the development roller 106, a hopper 111 containing a non-magnetic single-component toner 110 is held with a predetermined distance between a bottom opening of the hopper 111 and the surface of the development roller 106. A friction-charging member 112 is adhered to an inside surface of a right side wall of the hopper 111. A flattening member 113 is held on the right side of the development roller 106 in contact with the surface of the development roller 106 under pressure. The flattening member 113 includes a metal roller 114, a rubber layer 115 coating the surface of the metal roller 114, and a friction-charging member 116 coating the surface of the rubber layer 115. The flattening member 113 is held incapable of rotating.
With the above-described development roller 106, the toner can be carried thereon in a form of a thin layer. When the development roller 106 carrying the toner layer contacts the photosensitive drum 101 carrying an electrostatic latent image, the toner is transferred to the photosensitive drum 101 in accordance with a degree of an electric field for the development. Thereby, the electrostatic latent image formed on the photosensitive drum 101 is visualized with the toner. In using such a development roller 106, it is necessary to control a charge polarity and a charge amount relative to the toner with a friction charging through the contact between the toner and the development roller 106. Transfer of the toner to the photosensitive drum 101 is achieved by selections of an image region and a non-image region (a background region) in the electrostatic latent image. This background development apparatus illustrated in FIG. 15 advantageously facilitates an implementation of color image forming without the need of a non-magnet substance for the toner.
In the above-described background development apparatus, which develops an electrostatic latent image formed on the image carrying member with a single-component toner, a development system capable of performing the development at a relatively low development potential is needed. This is because there is a tendency to lower the charge potential in order to reduce an occurrence of a hazardous event during the charging time to achieve a long-life of a total image forming system including the development system, and particularly, of a photosensitive member. From a standpoint of achieving a high image quality, it becomes necessary to increase a development gamma as much as possible, which is a gradient of a curve representing a character of an image density relative to an electrostatic potential, and to make a development potential as small as possible for a saturation of the toner deposition to the photosensitive drum.
It is known to make a resistance of the development roller 106 smaller to raise the development gamma. When a resistance of the development roller is made small, it requires an application of a relatively high voltage such as xe2x88x92250 volts, for example, as a bias voltage, resulting in a problem of a current leakage to the image carrying member. Such leakage is prevented in the background development apparatus by an arrangement in that the resistance of the elastic layer 108 is made smaller than the resistance of the surface layer by adding a conductivity adding agent such as a carbon black to the elastic layer 108. However, when an excessive amount of the conductivity adding agent such as carbon black is added to the elastic layer, the hardness of the elastic layer is increased. This causes a problem of an inferior image quality since a nip is not sufficiently formed between the development roller 106 and the photosensitive drum 101. Also, it leads to another problem in that the elastic layer 108 is not easily formed in a roll shape.
This patent specification describes a novel development roller that addresses the above and other drawbacks in the background art. In one example, the novel development roller includes a core shaft, an elastic layer coating the core shaft, and a surface layer coating the elastic layer. The resistance of the surface layer is smaller than a resistance of the elastic layer, and an entire volume resistance of the development roller is smaller than a volume resistance of the elastic layer.
The volume resistance of the elastic layer may be 1.0xc3x97109 ohmxc2x7cm or less. The entire volume resistance of the development roller may be 1.0xc3x97107 ohmxc2x7cm or less. The resistance of the surface layer may be 1.0xc3x97108 ohms or less. The surface layer may have a thickness of 30 microns or less.
The patent specification further describes a novel development apparatus. In one example, the novel development apparatus includes a development roller that includes a core shaft, an elastic layer coating the core shaft, and a surface layer coating the elastic layer. The resistance of the surface layer can be smaller than a resistance of the elastic layer, and an entire volume resistance of the development roller can be smaller than a volume resistance of the elastic layer.
The patent specification further describes a novel image forming apparatus. In one example, the novel image forming apparatus includes a photosensitive member and a development mechanism including a development roller. The development roller includes a core shaft, an elastic layer coating the core shaft, and a surface layer coating the elastic layer. A resistance of the surface layer is smaller than a resistance of the elastic layer, and an entire volume resistance of the development roller is smaller than a volume resistance of the elastic layer.
The patent specification further describes a novel method of image forming. In one example, the novel method of image forming includes the steps of providing, supplying, regulating, and transferring. The providing step provides a rotary development roller that includes a core shaft coated with an elastic layer and a surface layer in this order, in parallel to a photosensitive member. The supplying step supplies toner onto a surface of the rotary development roller. The regulating step regulates the toner into a thin toner layer. The transferring step transfers the thin toner layer to the photosensitive member. A resistance of the surface layer is smaller than a resistance of the elastic layer, and an entire volume resistance of the development roller is smaller than a volume resistance of the elastic layer.
The volume resistance of the surface layer may be 1.0xc3x97109 ohmxc2x7cm or less. The entire volume resistance of the development roller may be 1.0xc3x97107 ohmxc2x7cm or less. The resistance of the surface layer may be 1.0xc3x97108 ohms or less. The surface layer may have a thickness of 30 microns or less. A toner deposition amount relative to a surface of the photosensitive member may be saturated when a development bias has a difference of 150 volts or less from a surface potential of an image region in the photosensitive member.